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They are either dumb or lying.
The UK governments own policy and strategy states that you shouldn't build web apps or indeed any apes which unnecessarily tie you to a browser, client or device. But of course this doesn't happen and so they are left in a mess. If they did things properly, they could use any modern browser.
The reality is that they have had circles run round them by IT vendors with no challenge from "unintelligent customers" who have no expertise in what makes for a good, maintainable, long lasting IT.
They're lying about saving money. They're hiding their incompetence.